The technology of the present disclosure relates to a motion assist device, which is worn on a body of a person, mainly an aged person, etc., who desires assistance and nursing care, to physically and psychologically assist motion of the person's body, and to a motion assist method, and particularly relates to a motion assist device and a motion assist method for generally assisting various motions of the person's body including walking motion.
Japan's population aging rate (a proportion of elderly persons aged 65 or over to the total population) was 23.1% in 2010, which is expected to reach 30% in 2025. With such a rapid increase in ratio of elderly persons in the population composition, it has become an urgent task to realize a society where elderly persons can live healthy and actively without falling into a condition of need for long-term care as far as possible, and even if they fell into the condition of need for long-term care, they are prevented from worsening as far as possible and can lead an independent life.
In the face of an aging society, there is a growing demand in nursing homes and households with elderly members for mechatronic devices intended to physically and mentally assist elderly persons. Beyond the physical assistance provided by an autonomous walking assist device, a power assist suit, etc., there is also a demand for mental assistance by an occupational therapy in which a robot is effectively incorporated.
One of the important considerations in the development of assist/nursing-care mechatronic devices is to maintain and promote activities of elderly persons to the extent possible without unnecessary interference. If activities of the elderly persons are excessively performed by the machine just because they have declined in strength, the strength of the elderly persons will further decline, making the situation worse (disuse syndrome). The power assist suit, which is a device that applies an artificial force as assistance to a force generated by human muscles, is a desirable device in that it can maintain activities of the elderly person while supplementing the decreased strength of the elderly person.
However, at present the penetration rate of the power assist suit is still low. The following are the probable reasons.
(1) The power assist suit is troublesome to wear;
(2) It is expensive;
(3) The device is heavy;
(4) It gives support only in an awkward manner;
(5) It looks clumsy when worn; and
(6) The operating time is short.
For example, a force-control-type power assist method, which applies a driving force to joints on the basis of outputs from a myoelectric sensor and estimation results of motion phases, has recently been drawing attention (see e.g., Kawamoto H., Lee S., Kanbe S., Sankai Y.: “Power Assist Method for HAL-3 using EMG-based Feedback Controller”, Proc. of Intl Conf. on Systems, Man and Cybernetics (SMC2003), pp. 1648-1653, 2003). However, since as much as nine myoelectric sensors have to be attached to one leg, they are troublesome to wean In addition, the myoelectric sensor can come off the skin due to temporal change or perspiration. Once the contact between the myoelectric sensor and the skin is lost, output values of the myoelectric sensor become unstable, which may cause the power assist suit to go out of control or an improper force to be applied to the body of the person wearing the suit.
Further, there has been proposed a walking assist device which applies a designed torque pattern in accordance with a phase of walking to a person's body while the person is walking (see e.g., Kenta Suzuki, Gouji Mito, Hiroaki Kawamoto, Yasuhisa Hasegawa, Yoshiyuki Sankai: “Intention-Based Walking Support for Paraplegia Patients with Robot Suit HAL”, Advanced Robotics, Vol. 21, No. 12, pp. 1441-1169, 2007). However, since users walk in various patterns, there are many cases which are not covered by the designed torque patterns. For this reason, the walking assist device is likely to cause a sense of discomfort during walking or be capable of only unnatural, low speed walking.
On the other hand, there has been also proposed a body assist device which uses no myoelectric sensors (e.g., see J. Chan, R. Steger, Kazerooni, H., “Control and System Identification for the Berkeley Lower Extremity Exoskeleton”, Advanced Robotics, Volume 20, Number 9, pp. 989-1011, Number 9, 2006). This device is configured to sense motion of a user's joint and apply a force for supporting the motion to the joint. However, this device unfortunately fails to reflect an intention of the user's motion with high sensitivity in the presence of an obstacle to the motion of the user's joint. For example, viscosity resistance at a gear part included in the joint units of the current power assist suit can cause an obstacle to the motion of the user's joint. Such an obstructive factor has to be eliminated in the future.
Most of the power assist suits which use no myoelectric sensors generate a force on the basis of an empirical rule or an invalid control law. Ideally, the myoelectric sensor can directly reflect an intention of the user's motion (although in reality, as mentioned above, stable sensing is hard to achieve with the myoelectric sensor). On the other hand, it is difficult to extract intentions of the user through sensing of the joint motion. Under these conditions, the present inventors consider that there has to be a valid control law for providing a supportive force without causing stress or a sense of unnaturalness to the user.
For example, there has been proposed a walking assist system which assists forward swing of a leg while supporting balance and a body weight of a user (see e.g., Japanese Patent Laid-Open No. 2011-62163). This walking assist system is constituted of an inverted-pendulum-type moving body for the user to grip and a walking assist device for assisting motion of the user's legs, and is configured such that a predetermined speed relationship is established between a target travelling speed of the inverted-pendulum-type moving body and that of the walking assist device. The inverted-pendulum-type moving body controls the travel on the basis of the target travelling speed and movement of a base body when the user grips the moving body, while the walking assist device transmits a force to the user on the basis of motion of the user's leg and the target travelling speed. Thus, the walking assist system can be considered to be a system which provides rhythm assistance, that is, supports a hip joint in accordance with a phase of walking. However, assistance of the walking assist system is applicable to walking motion and it is not versatile enough to be applied to other motions of the person's body. Additionally, a body weight assist having a saddle at a crotch part has also been proposed, but it gets in the way of sitting down and has an unattractive appearance.
The power assist suit in related art generally includes one actuator for driving of each joint. Accordingly, assisting more joint portions results in increasing the number of the actuators, which causes the device to become heavier, more expensive, and hence less practical. In addition, the high proportion of the actuators in the device restricts the design, so that the device tends to be visually unappealing, and contributes to shorter operating time due to increase of driving power.